Transformer oil composition, comprising at least one acid interceptor

ABSTRACT

The invention relates to a transformer oil composition comprising at least one acid interceptor. The invention further relates to the use of acid interceptor in transformer oils for improving stability.

The present invention relates to a transformer oil composition withimproved stability. The present invention further provides for the useof at least one acid scavenger as a stabilizer in transformer oilcompositions. The present invention further relates to the use of atleast one acid scavenger for improving the oxidation stability oftransformer oils. The present invention finally also relates to aprocess for producing corresponding inventive transformer oils.

A transformer is a component in electrical engineering which transferselectrical energy or information between inductively coupled circuitswith low loss. Viewed from the outside, a transformer consists of atransformer tank manufactured from sheet steel. Within the tank is theactive part of the transformer, namely the coils or windings thereof,applied to an iron core. The iron core consists of the limbs which bearthe windings, and the two yokes which complete the magnetic pathway. Theiron core consists of a coated magnetizable iron sheet. Eddy currentsand associative losses and heating are prevented since an insulatinglayer, for example of waterglass, of the sheets lies transverse to thedirection of the eddy current. For insulation reasons, the low-voltagewinding is on the inside and the high-voltage winding on the outside ofthe iron core. The windings consist of retainers wrapped withoil-impregnated paper.

All cavities of a transformer are filled with a transformer oil forelectrical insulation and for cooling of the coils.

Transformers are some of the most important and costly pieces ofequipment in electrical power supply. The occurrence of faults in thesecomponents therefore leads not only to interruptions in the electricalpower supply, but also to economic loss. The lifetime of a transformerdepends in particular on the insulation material, which is obtained andproduced from paper. These highly compacted papers, however, decomposein the course of operation of a transformer and form water. The effectsof this moisture on the insulation of old transformers have become animportant point of emphasis in the study of transformer failures.

A report published by the American company Hartford Steam BoilerInspection and Insurance Co. (HSB), which offers insurance againstequipment failure, arrived at the conclusion that damage to theinsulation in the last 10 years constituted the second most frequentcause of failure of transformers. The mean average age of thetransformers which failed due to damaged insulation was 17.8 years, andis thus well below the envisaged lifetime of 35 to 40 years.

The solid paper insulation used in transformers is usually based oncellulose. When cellulose ages, the polymer structure thereof decomposesand gradually releases water into the insulation oil, referred to astransformer oil. If the transformer fluid cannot absorb the water, itremains in the winding.

However, the occurrence of moisture is disadvantageous not only withregard to the reduced insulating action, since the water furtheraccelerates the aging of cellulose and in turn gives rise to water in anautocatalytic process. The destruction of the transformers increasesfurther as a result.

In “free-breathing” transformers, it is additionally possible for moremoisture to be absorbed from the atmosphere.

This reduction in quality of the transformer cellulose reduces both theelectrical and mechanical stability thereof. In general, the greater thewater content in the transformer oil, the greater the reduction in themechanical stability of transformer cellulose.

Mineral oil has only a very low capacity to absorb moisture. Themajority of the water which forms in the course of aging of transformercellulose remains in the windings and thus lowers the insulationresistance of the transformer. The moisture also reduces the resistanceof the transformer to the mechanical and electrical stresses which occurin operation.

In addition, a high moisture level can significantly lower thedielectric strength of the mineral oil. In time, this leads to failuresand/or to the necessity of reducing the power of the transformer, andcan ultimately cause complete failure of the transformer.

The decomposition of cellulose in transformers is catalyzed by thepresence of short-chain fatty acids. The short-chain fatty acids forminitially as a result of decomposition from the cellulose used for theproduction of the papers. The higher the level of fatty acids present inthe transformer oil composition, the more rapidly the fatty aciddecomposes. Viewed in chemical terms, this process is thusautocatalytic. When the decomposition is advanced and causes paperfracture in a transformer, the transformer can even explode in the worstcase.

Furthermore, the oils used in the transformers, called transformer oils,are expensive and should therefore be reused in the case of a loss ofquality due to the formation of water and/or fatty acid. In thereprocessing of transformer oils, substances such as water and sedimentsare generally removed, and the transformer oil is typically heated up ina special machine, called an oil separation machine, and dried underreduced pressure by spraying. In the course of this, the transformer oilis filtered and freed of the existing sediments.

Irrespective of these reprocessing options, there is a great interest inimproving the stability of transformer oils such that the lifetime ofthe transformer oil is prolonged overall, and reprocessing oftransformer oils is required at a later stage or not at all.

The object of this invention is achieved by a composition whichcomprises at least one transformer oil and, as an additive, at least oneacid scavenger. In the context of the present invention, the acidscavengers used may especially be carbodiimides; epoxidizedtriglycerides, such as epoxidized soybean oil and epoxidized rapeseedoil; trialkylamines; dialkylamines; pyridines or pyridine derivatives;diazabicyclooctane; and polyvinylpyridine. In a preferred embodiment ofthe present invention, the acid scavenger is at least one carbodiimidecompound. The carbodiimide compound is preferably a monomeric, dimericor polymeric carbodiimide.

It has been found in accordance with the invention that the addition ofat least one acid scavenger, especially of at least one carbodiimidecompound, leads to elimination of fatty acids already formed intransformer oils, which—as explained above—lead to catalysis ofcellulose decomposition and hence to unwanted formation of water.Therefore, the addition of at least one acid scavenger, such as at leastone carbodiimide compound, leads to an interruption in the autocatalyticdecomposition of cellulose in a transformer, and hence also to areduction in water formation in transformer oil compositions. The acidscavengers, such as carbodiimides, in the context of the presentinvention, thus stabilize the transformer oil compositions and henceincrease the lifetime of the transformer and the service life of thetransformer oil compositions. The acid scavengers provided in accordancewith the invention, such as the carbodiimides, react selectively withfatty acids. They thus reduce the content of fatty acids in the agedtransformer cellulose, which in turn catalyze the decomposition of thecellulose to water.

In the context of the present invention, an acid scavenger is thereforeunderstood to mean any compound which is capable of suppressing theformation of fatty acid from cellulose, or is capable of at leastkeeping the concentration of fatty acid in a transformer oil constant,preferably of reducing it.

In the context of the present invention, stabilization of a transformeroil composition is especially understood to mean that the electricallyinsulating properties and the heat-removing properties of thetransformer oil composition are maintained for longer than in the caseof no additization.

More particularly, stabilization of a transformer oil composition by anacid scavenger is understood to mean that the acid scavenger, such asthe carbodiimide, is capable of scavenging the fatty acids present inthe composition, and of keeping the water content in the compositionconstant, preferably of reducing it.

In addition, stabilization of a transformer oil composition by an acidscavenger is also understood to mean that the transformer constituentsare protected from corrosion.

The carbodiimide acid scavengers particularly preferred in accordancewith the invention are first described in detail hereinafter.

Carbodiimide Compounds

The requirements on a carbodiimide compound as the inventiveadditization relate firstly to the solubility thereof in the transformeroils, which may, for example, be ester base oils or mineral oils. Thecarbodiimide compounds preferably have, at a temperature of 60° C., asolubility in the corresponding transformer oils of 0.001 to 2.5% bymass, more preferably 0.1 to 1% by mass, especially 0.2 to 0.5% by mass.

In addition, the carbodiimide compound should have a long-term stabilityin the transformer oil composition. The long-term stability should existunder the conditions of use of transformer oils, i.e. especially in thepresence of moisture and at temperatures at which the transformer oilsare also used. It is therefore especially preferred when thecarbodiimide compound to be used in accordance with the invention has along-term stability in a transformer oil which is used at 60° C. and hasa water content of preferably at most 30 ppm in the case of mineral oilsand 400 ppm in the case of ester-based insulating oils.

Furthermore, the carbodiimide compounds should be such that they reactrapidly and selectively with short-chain acids, especially with lacticacid and/or butyric acid.

The carbodiimide compound to be used with preference in accordance withthe invention, with regard to the chemical structure thereof, is notsubject to any particular requirement at first, provided that thecarbodiimide is capable of stabilizing the transformer oil compositionfor the purposes of the present invention, i.e. more particularly iscapable of reacting with the fatty acids present in the transformer oil.

The carbodiimide compounds which can be used in the present inventionmay be those which are synthesized by commonly known processes. Thecompound can be obtained, for example, by means of performance of adecarboxylation condensation reaction of different polyisocyanates usingan organophosphorus compound or an organometallic compound as a catalystat a temperature of not less than about 70° C. without using any solventor using an inert solvent.

Examples of a monocarbodiimide compound which is present in theabove-described carbodiimide compounds are dicyclohexylcarbodiimide,diisopropylcarbodiimide, dimethylcarbodiimide, diisobutylcarbodiimide,dioctylcarbodiimide, tert-butylisopropyl-carbodiimide,2,6-diisopropylphenylenecarbodiimide, diphenylcarbodiimide,di-tert-butylcarbodiimide and di-β-naphthylcarbodiimide, particularpreference being given to dicyclohexylcarbodiimide ordiisopropylcarbodiimide with regard to industrial employability.

Corresponding processes for preparing carbodiimides and correspondinglysuitable compounds are described, for example, in U.S. Pat. No.2,941,956, JP-B-47-33279, J. Org. Chem. 28, 2069-2075 (1963) andChemical Review, 1981, Vol. 81, No. 4, pages 619 to 621.

An organic diisocyanate as a starting material for preparing apolycarbodiimide compound includes, for example, aromatic diisocyanate,aliphatic diisocyanate, alicyclic diisocyanate or a mixture thereof, andincludes especially naphthalene 1,5-diisocyanate, diphenylmethane4,4′-diisocyanate, diphenyldimethylmethane 4,4′-diisocyanate, phenylene1,3-diisocyanate, phenylene 1,4-diisocyanate, tolylene 2,4-diisocyanate,tolylene 2,6-diisocyanate, a mixture of tolylene 2,4-diisocyanate andtolylene 2,6-diisocyanate, hexamethylene diisocyanate, cyclohexane1,4-diisocyanate, xylylene diisocyanate, isophorone diisocyanate,dicyclohexylmethane 4,4′-diisocyanate, methylcyclohexane diisocyanate,tetramethylxylylene diisocyanate, 2,6-diisopropylphenylene isocyanateand 1,3,5-triisopropylbenzene 2,4-diisocyanate.

In addition, in the above-described polycarbodiimide compound, thedegree of polymerization thereof can be controlled to a suitable levelusing a compound, such as monoisocyanate, which is capable of reactionwith a terminal isocyanate group of the polycarbodiimide compound.

The monoisocyanate for control of the degree of polymerization byprotection of a terminal group of the polycarbodiimide compound includesphenyl isocyanate, tolyl isocyanate, dimethylphenyl isocyanate,cyclohexyl isocyanate, butyl isocyanate and naphthyl isocyanate.

In addition, the terminal protecting agent for controlling the degree ofpolymerization by protecting a terminal group of the polycarbodiimidecompound is not limited to the above-described monoisocyanates, andincludes compounds with an active hydrogen capable of reaction withisocyanate, for example (i) an aliphatic, aromatic or alicyclic compoundwith an —OH group, such as methanol, ethanol, phenol, cyclohexanol,N-methylethanolamine, oligo-, polyethylene glycol monomethyl ether andoligo-, polypropylene glycol monoalkyl ethers, fatty alcohols and oleylalcohols; (ii) a compound with an ═NH group, such as diethylamine anddicyclohexylamine; (iii) a compound with an ═NH₂ group, such asbutylamine and cyclohexylamine; (iv) a compound with a —COOH group, suchas succinic acid, benzoic acid and cyclohexanecarboxylic acid; (v) acompound with an —SH group, such as ethyl mercaptan, allyl mercaptan andthiophenol; and (vi) a compound with an epoxy group.

The decarboxylation condensation reaction of the above-described organicdiisocyanate is performed in the presence of a suitable carbodiimidationcatalyst. Preferred carbodiimidation catalysts for use are anorganophosphorus compound and an organometallic compound [a compoundexpressed by the general formula M-(OR)₄ in which M is titanium (Ti),sodium (Na), potassium (K), vanadium (V), tungsten (W), hafnium (Hf),zirconium (Zr), lead (Pb), manganese (Mn), nickel (Ni), calcium (Ca) andbarium (Ba) and the like; R is an alkyl group or aryl group having 1 to20 carbon atoms], and particular preference from an activity point ofview is given to phospholene oxide among the organophosphorus compounds,and to alkoxides of titanium, hafnium and zirconium among theorganometallic compounds. Mention should additionally be made of strongbases, for example alkali metal or alkaline earth metal hydroxides oroxides, alkoxides and phenoxides.

The above-described phospholene oxides include especially3-methyl-1-phenyl-2-phospholene 1-oxide, 3-methyl-1-ethyl-2-phospholene1-oxide, 1,3-dimethyl-2-phospholene 1-oxide, 1-phenyl-2-phospholene1-oxide, 1-ethyl-2-phospholene 1-oxide, 1-methyl-2-phospholene 1-oxideand double bond isomers thereof. Among these, owing to easy industrialavailability, particular preference is given to3-methyl-1-phenyl-2-phospholene 1-oxide.

The carbodiimide compound is especially4,4′-dicyclohexylmethanecarbodiimide (degree of polymerization=2 to 20),tetramethylxylylenecarbodiimide (degree of polymerization=2 to 20),N,N-dimethylphenylcarbodiimide (degree of polymerization=2 to 20) andN,N′-di-2,6-diisopropylphenylenecarbodiimide (degree of polymerization=2to 20) and the like, and is not especially restricted provided that thecompound has at least one carbodiimide group in a molecule with thefunction.

Carbodiimides suitable in the context of the present invention areespecially monomeric, dimeric or polymeric carbodiimides.

Several preferred configurations of the carbodiimide compound areexplained in detail hereinafter.

In a first configuration, in the context of the present invention, amonomeric carbodiimide is used.

In this configuration of the present invention, the carbodiimidepreferably has the general formula (I)

in which the R¹ to R⁴ radicals are each independently a straight-chainor branched C₂- to C₂₀-alkyl radical, a C₂- to C₂₀-cycloalkyl radical, aC₆- to C₁₅-aryl radical or a C₆- to C₁₅-aralkyl radical.

For the R¹ to R⁴ radicals, preference is given to C₂-C₂₀-alkyl and/orC₂-C₂₀-cycloalkyl radicals.

For the R¹ to R⁴ radicals, particular preference is given toC₂-C₂₀-alkyl radicals.

C₂-C₂₀-Alkyl and/or C₂-C₂₀-cycloalkyl radicals are understood in thecontext of the present invention to mean especially ethyl, propyl,isopropyl, sec-butyl, tert-butyl, cyclohexyl and dodecyl radicals,particular preference being given to the isopropyl radical.

C₆- to C₁₅-Aryl and/or C₆- to C₁₅-aralkyl radicals are understood in thecontext of the present invention to mean especially phenyl, tolyl,benzyl and naphthyl radicals.

A corresponding carbodiimide is commercially available from Rhein ChemieRheinau GmbH under the names Additin® 8500, Stabaxol® 1 I or Stabaxol® ILF. The products sold by Raschig with the name Stabilizer 3000, 7000 and7000 A can also be used in the context of the present invention.

In a second configuration, in the context of the present invention, apolymeric carbodiimide is used.

A corresponding polymeric carbodiimide has the general formula (II)

R′—(—N═C═N—R—)_(n)—R″  (II)

in which

-   -   R is an aromatic, aliphatic, cycloaliphatic or araliphatic        radical which, in the case of an aromatic or araliphatic radical        in one ortho position, preferably in both ortho positions, to        the aromatic carbon atom which bears the carbodiimide group, may        bear aliphatic and/or cycloaliphatic substituents having at        least 2 carbon atoms, preferably branched or cyclic aliphatic        radicals having at least 3 carbon atoms, especially isopropyl        groups,    -   R′ is aryl, aralkyl or R—NCO, R—NHCONHR¹, R—NHCONR¹R² and        R—NHCOOR³,    -   R″ is —N═C═N-aryl, —N═C═N-alkyl, —N═C═N-cycloaliphatic,    -   —N═C═N-aralkyl, —NCO, —NHCONHR¹, —NHCONR¹R² or NHCOOR³,    -   where, in R′ and in R″ independently, R¹ and R² are the same or        different and are each an alkyl, cycloalkyl or aralkyl radical        and R³ has one of the definitions of R¹, or is a polyester or        polyamide radical, and    -   n is an integer from 1 to 5000, preferably from 1 to 500.

In a first preferred embodiment of these polymeric carbodiimides, R isan aromatic or araliphatic radical which may bear, in at least one orthoposition, preferably in both ortho positions, to the aromatic carbonatom which bears the carbodiimide group, aliphatic and/or cycloaliphaticsubstituents with at least 2 carbon atoms, preferably branched or cyclicaliphatic radicals with at least 3 carbon atoms, especially isopropylgroups.

Particularly suitable carbodiimides are those of the general formulae(I) and (II) which are substituted by isopropyl in the ortho positionson this aromatic or araliphatic radical to the carbodiimide group, andwhich are likewise substituted by isopropyl in the para position to thecarbodiimide group.

In a second preferred embodiment of these polymeric carbodiimides, R isan aromatic radical which is joined via a C₁- to C₈-alkyl radical,preferably C₁- to C₄-alkyl radical, to the carbodiimide group (—N═C═N—).

In addition, it is also possible to use polymeric aliphaticcarbodiimides, for example based on isophorone diisocyanate or H12-MDI(hydrogenated MDI), which are obtainable from Nishinbo.

To prepare the carbodiimides and/or polycarbodiimides of the generalformula (I) or (II), it is possible to use monoisocyanates and/ordiisocyanates which are converted by condensation with elimination ofcarbon dioxide at elevated temperatures, for example at 40° C. to 200°C., in the presence of catalysts, to the corresponding carbodiimides.Suitable processes are described in DE-A-11 30 594 and in FR 1 180 370.Useful catalysts have been found to be, for example, strong bases orphosphorus compounds.

Preference is given to using phospholene oxides, phospholidines orphospholine oxides, and also the corresponding sulfides. In addition,the catalysts used may also be tertiary amines, basic metal compounds,carboxylic acid metal salts and nonbasic organometallic compounds.

All isocyanates are suitable for preparation of the carbodiimides and/orpolycarbodiimides used, preference being given in the context of thepresent invention to using carbodiimides and/or polycarbodiimides basedon C₁- to C₄-alkyl-substituted aromatic isocyanates, for example2,6-diisopropylphenyl isocyanate, 2,4,6-triisopropylphenyl1,3-diisocyanate, 2,4,6-triethylphenyl 1,3-diisocyanate,2,4,6-trimethylphenyl 1,3-diisocyanate,2,4′-diisocyanatodiphenylmethane,3,3′,5,5′-tetraisopropyl-4,4′-diisocyanatodiphenylmethane,3,3′,5,5′-tetraethyl-4,4′-diisocyanatodiphenylmethane, tetramethylxylenediisocyanate or mixtures thereof, and on substituted aralkyls, such as1,3-bis(1-methyl-1-isocyanatoethyl)benzene. It is particularly preferredwhen the carbodiimides and/or polycarbodiimides are based on2,4,6-triisopropylphenyl 1,3-diisocyanate.

When they have been prepared from isocyanates, polycarbodiimides mayalso still contain reactive NCO groups and complex-bound monomericisocyanates.

These polycarbodiimides containing NCO groups can be modified in such away that the isocyanate groups present are eliminated with reactive,hydrogen-containing compounds such as alcohols, phenols or amines (cf.DE 11 56 401 A and DE 24 19 968 A). In this regard, more particularly,reference should also be made to the use of polypropylene glycolmonoalkyl ethers, and fatty alcohol and oleyl alcohol residues for endcapping.

The polymeric carbodiimides of the general formula (II) may also beend-capped with isocyanate compounds.

Polymeric carbodiimides in the context of the present invention arecommercially available from Rhein Chemie Rheinau GmbH under the namesStabaxol® P, Stabaxol® P100, Stabaxol® P200 and Stabaxol P400. It isalso possible to use the products sold by Rasching with the nameStabilizer 2000, 9000 and 11000 in the context of the present invention.

The carbodiimides can also be used in the context of the presentinvention as a mixture of several carbodiimides.

In a further embodiment of the present invention, it is also possiblethat a mixture of different carbodiimides is used. When a mixture ofcarbodiimides is used, the carbodiimides used may be selected from thegroup of the monomeric, dimeric or polymeric carbodiimides, referencebeing made to the above remarks regarding the compounds of the generalformulae (I) and (II) with regard to the monomeric carbodiimides and thepolymeric carbodiimides.

In addition, it is preferred in the context of the present inventionwhen carbodiimides which contain a reduced content of free isocyanatesare used. Preferred carbodiimides contain preferably less than 1% byweight of free isocyanates.

In addition, it is possible also to use other acid scavengers as well ascarbodiimides.

The inventive transformer oil composition may comprise the at least onecarbodiimide in a wide range of variation of the amount added. Forreasons of efficacy, it is, however, preferred that the at least onecarbodiimide is present in the composition in an amount of at least0.01% by weight, based on the inventive transformer composition. Foreconomic reasons, there will be no interest in the amount of the atleast one carbodiimide in the transformer oil composition exceeding 10%by weight, based on the inventive transformer composition.

In a particularly preferred embodiment of the present invention, theamount of the at least one carbodiimide in the inventive composition is0.01 to 10% by weight, more preferably 0.05 to 5% by weight, especially0.1 to 1.5% by weight, based in each case on the inventive transformercomposition.

It is also possible to use the acid scavenger, especially thecarbodiimide, as a masterbatch, in which case the acid scavenger may bedissolved in a high concentration in a transformer oil and is diluted inapplication.

There now follows a more detailed description of the transformer oilwhich preferably finds use in the inventive composition:

Transformer Oils

In the context of the present invention, it is possible to use anytransformer oil. It is preferred when the transformer oil meets thespecification IEC 61099: 1992. This document “Specifications for unusedsynthetic organic esters for electrical purposes” is a general standardin which the demands on new synthetic organic esters for electronicpurposes are defined.

In general, the transformer oil is selected from the group consisting ofoils based on mineral oil, oils based on esters and oils based ontriglycerides.

When the transformer oils used are mineral oils, the mineral oils arepreferably mobile naphthenic low-sulfur base oils of group III accordingto the ATIEL-API classification of base oils, with good low-temperatureand oxidation properties. Corresponding mineral oils are available, forexample, from Nynaes (Sweden) under the trade name Nytro® or from Shellunder the trade name Shell Diala D. These transformer oils meet IEC60296: 2003—“Fluids for electrotechnical applications—Unused mineralinsulating oils for transformers and switchgears”.

When there is a requirement for transformer oils which have lowflammability and are biodegradable and exhibit a relatively high waterabsorption, preference is given to using transformer oils based onbranched or linear saturated mobile polyol esters with preferably lowacid numbers. Examples thereof are the systems sold by M & J under thetrade name Midel®. Also suitable in this case are soybean oil, rapeseedoil or sunflower oil.

In a preferred embodiment of the present invention, the transformer oilis a trimethylolpropane ester (TMP) of the general formula (III)

Corresponding trimethylolpropane esters are known from German patentapplication DE 10 2004 025 939 A1. In the above general formula (III),the R¹, R² and R³ radicals are the same or different and are each alinear or branched alkyl group having 5 to 11 carbon atoms. In a furtherpreferred embodiment of the present invention, the R¹, R² and R³radicals are the same or different and are each a linear or branchedalkyl group having 7 to 9 carbon atoms.

The trimethylolpropane ester (IMP) of the general formula (III) featuresa viscosity of less than 23 mm²/s at 40° C. and a combustion point ofhigher than 300° C. These esters are thus outstandingly suitable as adielectric insulating fluid for transformers.

The low viscosity can be achieved by selected acid components in theesterification. The R¹, R² and R³ radicals in the formula (I) consist oflinear or branched alkyl groups having 5 to 11 carbon atoms. Preferenceis given to the use of radicals with linear or branched alkyl groupshaving 7 to 9 carbon atoms. The radicals must be saturated in order toachieve the oxidation stability necessary. It is possible for allradicals in a polyol ester to be the same, for only two radicals to bethe same or for all of them to be different. Preference is given to adistribution of radicals with 7 to 9 carbon atoms which arise in theesterification of trimethylolpropanol with an acid mixture of C₈- toC₁₀-fatty acids, where the combustion point must be above 300° C. andthe viscosity attains the preferred ranges described, of less than 23mm²/s at 40° C. The higher the number of carbon atoms, the higher thecombustion point will be, but the higher the viscosity will be. Sincethese values run in opposite directions, there is an optimum carbonchain length distribution of the R¹, R² and R³ radicals for each pair ofvalues.

This class of trimethylolpropane esters meets the demands of standardIEC61099 and they are classified by the German Federal EnvironmentAgency (UBA, Berlin) as not hazardous to water (NWG).

In a further embodiment of the present invention, the transformer oilmay also be a pentaerythrityl ester. Such ester-based transformer oilsare known on the market, for example, as Midel® 7131. In unadditizedform, these transformer oils, however, lead to acid formation frominsulation paper and thus lead to formation of water, which in turnleads to severe damage in transformers.

In addition to the already mentioned stabilizing effect due to the acidscavenger action of the carbodiimides in transformer oils, thecarbodiimides also act synergistically in relation to copper, lead, tinand zinc, by preventing the corrosion of these metals used in thetransformers and thus protecting the transformer oils in contact withthese metals from aging. This inventive synergistic effect occursespecially when there are defects in the paper insulation and thewindings come into contact with the transformer oil. This inventivesynergistic effect is advantageous especially in the case of windingswhich comprise copper as material.

The inventive transformer oil composition may additionally comprisefurther additives customary for this field of use. For example, thesemay be antioxidants or metal deactivators.

In a further embodiment, the inventive composition thereforeadditionally contains 0.005 to 1.0% by weight of an antioxidant and/or0.01 to 2.0% by weight of a metal deactivator, based in each case on thetransformer oil.

The preferred amount of antioxidant is between 0.1 and 0.5% by weightand is especially 0.1% by weight, based on the transformer oil.

The preferred amount of metal deactivator is between 0.1 and 1.0% byweight and is especially 0.1% by weight, based on the transformer oil.

The antioxidant is preferably selected from the group consisting ofbishydroxytoluene, hydroquinone, 4-tert-butylcatechol, naphthol,phenylnaphthylamines, diphenylamines, phenylic thioethers, tocopherolsand mixtures of the substances listed. A suitable antioxidant isespecially 2,6-di-tert-butylhydroxytoluene (BHT), which is sold underthe Baynox® trade name by Lanxess Deutschland GmbH.

The metal activator is preferably selected from the group consisting oforganic hetero compounds such as triazoles, tolyltriazoles,dimercaptothiadiazoles and mixtures of the substances listed.

The present invention additionally relates to a process for producing acorresponding inventive transformer oil composition by mixing thetransformer oil with at least one acid scavenger, especially the atleast one carbodiimide. For this purpose, the acid scavenger is added tothe transformer oil, the resulting composition is optionally heated andthe acid scavenger is stirred into the transformer oil with standardequipment. At the customary operating temperature of transformer oils(˜60° C.), the acid scavenger is generally likewise dissolved in thetransformer oil, and so it is also possible to dispense with heatingand/or stirring of the additive into the transformer oil before the usethereof.

The present invention additionally relates to the use of at least oneacid scavenger, especially a carbodiimide, as a stabilizer intransformer oil compositions. With regard to the carbodiimide and thetransformer oil composition, reference is made to the above remarks.

The present invention additionally relates especially to the use of atleast one carbodiimide as an acid scavenger in transformer oilcompositions. With regard to the description of the at least onecarbodiimide and of the transformer oil, reference is made to the aboveremarks.

The present invention further relates especially to the use of at leastone carbodiimide for protection of transformers from corrosion. Withregard to the description of the at least one carbodiimide and of thetransformer oil, reference is made to the above remarks.

The carbodiimides provided in accordance with the invention can be addedeither to new transformer oils which are being used for the first timeor to transformer oils which are already in use. More particularly, itis also possible to add the acid scavengers provided in accordance withthe invention, especially the carbodiimides, as additives to alreadyregenerated transformer oils.

Corresponding processes for regenerating used transformer oils are knownper se to those skilled in the art, and reference is made to theextensive known prior art on this subject.

The present invention further provides transformers which comprise theabove-described transformer oil composition. The transformers may bepower transformers, distribution transformers, mast transformers, tapchangers or changeover switches.

The invention is illustrated by the examples below, without beingrestricted thereto.

WORKING EXAMPLES

The following samples were examined in relation to the agingcharacteristics:

Sample 1: 100% by weight of Shell Diala D (comparative example) and

Sample 2: 100% by weight of Shell Diala D+1% Additin® RC 8500 (acarbodiimide of the formula I where R₁, R₂, R₃ and R₄=isopropyl,obtainable from Rheinchemie Rheinau GmbH) as an inventive example.

Samples 1 and 2 were aged at 145° C. in the presence of 20 weight ofinsulating paper (from Weidmann Plastics Technology AG) in a closedvessel over a period of 40 days. The aging-associated depolymerization(determined to DIN EN 60450) of the insulating paper was taken as ameasure for the advancing aging. FIG. 1 shows the results in graphicform. The degree of polymerization was measured over time. It is evidentthat the addition of carbodiimide slows the depolymerization of theinsulating paper from the start.

What is claimed is:
 1. A composition comprising at least one transformeroil and at least one acid scavenger.
 2. The composition as claimed inclaim 1, characterized in that the at least one acid scavenger isselected from the group consisting of carbodiimides; epoxidizedtriglycerides; trialkylamines, dialkylamines; pyridines and pyridinederivatives; diazabicyclooctane; and polyvinylpyridine.
 3. Thecomposition as claimed in claim 2, characterized in that the acidscavenger is at least one carbodiimide.
 4. The composition as claimed inclaim 3, characterized in that the at least one carbodiimide is amonomeric carbodiimide of the general formula (I)

in which the R¹ to R⁴ radicals are each independently a straight-chainor branched C₂- to C₂₀-alkyl radical, a C₂- to C₂₀-cycloalkyl radical, aC₆- to C₁₅-aryl radical or a C₆- to C₁₅-aralkyl radical.
 5. Thecomposition as claimed in claim 3, characterized in that the at leastone carbodiimide is a carbodiimide of the general formula (II)R′—(—N═C═N—R—)_(n)—R″  (II) in which R is an aromatic, aliphatic,cycloaliphatic or araliphatic radical which, in the case of an aromaticor araliphatic radical in one ortho position, preferably in both orthopositions, to the aromatic carbon atom which bears the carbodiimidegroup, may bear aliphatic and/or cycloaliphatic substituents having atleast 2 carbon atoms, preferably branched or cyclic aliphatic radicalshaving at least 3 carbon atoms, especially isopropyl groups, R′ is aryl,aralkyl or R—NCO, R—NHCONHR¹, R—NHCONR¹R² and R—NHCOOR³, R″ is—N═C═N-aryl, —N═C═N-alkyl, —N═C═N-cycloaliphatic, —N═C═N-aralkyl, —NCO,—NHCONHR¹, —NHCONR¹R² or NHCOOR³, where, in R′ and in R″ independently,R¹ and R² are the same or different and are each an alkyl, cycloalkyl oraralkyl radical and R³ has one of the definitions of R¹, or is apolyester or polyamide radical, and n is an integer from 1 to 5000,preferably from 1 to
 500. 6. The composition as claimed in any of claims3 to 5, characterized in that the carbodiimides are used as mixtures. 7.The composition as claimed in any of claims 3 to 6, characterized inthat the amount of the at least one carbodiimide in the composition is0.01 to 10% by weight, more preferably 0.05 to 5% by weight, especially0.1 to 1.5% by weight.
 8. The composition as claimed in any of claims 1to 7, characterized in that the transformer oil is selected from oilsbased on mineral oil, based on esters or based on triglycerides.
 9. Thecomposition as claimed in any of claims 1 to 8, characterized in thatthe transformer oil is a trimethylolpropane ester (TMP) of the generalformula (III)

where R¹, R² and R³ are the same or different and are each a linear orbranched alkyl group having 5 to 11 carbon atoms.
 10. The composition asclaimed in any of claims 1 to 9, characterized in that the transformeroil is a pentaerythrityl ester.
 11. The use of at least one acidscavenger as defined in claim 2 for protection of transformers fromcorrosion or as a stabilizer in transformer oils.
 12. The use of atleast one carbodiimide corresponding to the definition according to anyof claims 3 to 5 as an acid scavenger.
 13. A process for producing acomposition as claimed in any of claims 1 to 10, characterized in that atransformer oil is mixed with at least one acid scavenger.
 14. Atransformer comprising a composition as claimed in any of claims 1 to10.